1. Field of the Invention
The present invention is related to an apparatus for passing a printed web between the separated cylinders of a deactivated printing unit of a web fed rotary printing press.
2. State of the Art
For producing high quality, high volume multicolor printed products, web fed rotary printing presses are used with two or more (e.g., four) printing units, each applying a different color to an endless web running through the printing press. When processing products which do not require the use of all printing units, (e. g., if only one color is used for a product, but the printing press comprises four printing units), it is desired to switch off the unused printing units in order to save costs and energy and to reduce the noise which is produced by the printing press.
It is possible to move the blanket or corresponding blanket cylinders which transfer the image from the plate cylinders onto the web, away from the web within the unused printing unit. However, there are several reasons why it is not easily possible to simply move the blanket cylinders away from each other and switch off the printing unit, so that the web which was already printed in upstream printing units can pass through the separated blanket cylinders of a motionless press. For example, a first reason is that the blanket cylinders are arranged inclined relative to the vertical such that an axis extending through the centers of two adjacent blanket cylinders is inclined towards the vertical by an angle of inclination of, for example, 10.degree. to 15.degree. while the web runs horizontally. Inclining the cylinders has the advantage that the web not only contacts the cylinders on a small contacting line, but wraps around a certain circumferential portion of each of the blanket cylinders, to stabilize the web and create a better pinch point. In addition, the inclination of the cylinders provides a consistent "peal off" of the web from the blanket cylinders.
A second reason it is not easily possible to simply move the blanket cylinders away from one another is that in most of the web fed printing presses in which the web is printed by two corresponding blanket cylinders on both sides, one of the blanket cylinders is usually immovably arranged in the printing unit in order to serve as a reference point, and only the other cylinder is moved when a separation of the cylinders is desired. Furthermore, due to the construction of the mechanical driving mechanism for moving the blanket cylinders apart from each other, it is often not possible to separate the cylinders more than about 2 mm to 4 mm from each other.
A third reason it is not easily possible to simply move the blanket cylinders away from one another is that the running web is subject to strong vibrations and fluttering. Thus, even if a gap of about 4 mm would be large enough to theoretically allow a free passing of the web through the separated inclined cylinders, the wet printed web would inevitably touch the surface of the cylinders, thereby dramatically reducing printing quality and increasing the danger of web breaks.
In order to deactivate one or more printing units in a lithographic printing press used for the production of high quality prints, the ink flow from the ink trays to the plate cylinders of a respective printing unit is interrupted, the printing plates on the plate cylinders are replaced by blank plates and the plate and blanket cylinders are afterwards driven at the usual web speed, whereby the web is in contact with the respective blanket cylinder and the dampening system is switched on. Alternately, the web can be diverted around the deactivated printing unit or printing units using, for example, air-operated blower bars or angle bars. However, using a diversion or by-passing of the printed web around the deactivated printing units has the disadvantage that for every new printing job in which the deactivated printing units have to be activated again, the endless web has to be threaded in the respective printing unit. Besides this, the diversion of the web around the deactivated printing unit is likely to cause a misregistering of the web, particularly if non-marking web floatation devices such as air bars are used. Further, the use of a diversion also requires a retiming of the printing units when they are removed from the web path.
Another possible technique to pass the web between the separated cylinders of a deactivated, motionless printing unit, is to guide the web over rigidly mounted grater rollers which are located upstream and downstream of a deactivated printing unit. However, grater rollers can produce unacceptable damage to not only the wet surface of a printed web, but also to the surface of an unprinted web in a lithographic printing press. Further, grater rollers can significantly increase the danger of web breaks as a result of their rigid mounting. Thus, grater rollers are typically only used in low quality newspaper printing presses.
U.K. Patent Specification 1 461 572 describes an apparatus for assisting in the guidance of the leading end of a moving flexible web which includes two curved arrangements of air nozzles located above and below the web path and arranged to converge towards the nip of cutter rollers of a subsequent cutting section. The jets of air produced by the air nozzles are directed towards the web path and are preferably concentrated towards the nip. The apparatus is only intended for use in threading a flexible web through the nips of a printing press or an auxiliary device, such as a cutter, a folder or a slitter which is not in operation.
German patent application DE-OS 28 22 137 describes an apparatus similar to that of the U.K. Patent Specification which serves as a threading and guiding aid for assisting in guidance of the leading end of a moving flexible web into the nip formed between two cutting cylinders of a subsequent cutting section. The apparatus comprises two blast air nozzles which blow air in a direction towards the nip, thereby centering the leading end of the paper web and preventing the web from moving sideward.